Telephone switching apparatus.



No. 657,3l5. Patented Sept. 4, I900. D. S. HULFISH. TELEPHONE SWITCHINGAPPARATUS.

(Application filed Apr. 12, 1900.)

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Patented Sept. 4, I900 D. S. HULFISH.

TELEPHONE SWITCHING APPARATUS. A licaciml filed Apr. 12. 1900.;

'(No Model.) 2 Sheets- Sheet 2.

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DAVID IIULFISH, OF CHICAGO, ILLINOIS, ASSIGNOR TO THE AMERICANBELL'TELEPHONE COMPANY, OF BOSTON, MASSACHUSETTS;

TELEPHONE SWITCHING APPARATUS.

SPECIFICATION forming part of Letters Patent No. 657,315, datedSeptember 4, 1900.

Application filed April 12, 1900.

which are connected to one another by trunk or toll circuits, and owingto the rapid development of telephonic service and to the improvementsmade in the systems employed it frequently happens that some of thesubstation and the toll circuits are provided with magneto-generators tooperate call-bells and electromagnetic line-signals, while othersubstation and toll circuits are equipped with means such as acentralized or common battery for automatically operating bells andsignals. When such is the case, both kinds of circuits may have theirswitching terminals and line-signals upon the samekeyboardsections atthe central stations, and in order that the operators may make theswitching connections between the terminals of the same or the diversekind of circuits two or more kinds of linking or cord circuits have hadto" be furnished.

The present invention relates to special means embodied in linking orcord circuits sible to connect two common-battery circuits with eachother, a common-battery circuit with magneto substation or toll circuit,or two magneto-circuits with eachother, and to inductively unite the twoplugs of a cord-circuitby the introduction of a repeating-coil in orderthat two connected circuits of the same kind may repeat into each other,and also that a magneto-circuit may be enabled Serial No. 125603. (Nomodel.)

to repeat into a common-battery circuit, and 'vice versa. i

In carrying out the invention the magneto. generator circuits areprovided at the central station with socket-switches and a line-signaland the test-rings of the sockets are grounded, and the com mom-batterycircuits are provided with socket-switches, a line-signal, and also.with a cut-off relay, all in the usual manner, "the apparatus at thestations at the opposite ends of both kinds of circuits being of theordinary character. It is necessary in order to effect the combinationof circuits indicated that a cord-circuit shall be provided havingdisconnecting or supervisory signals adapted for each class or kind ofcircuit with means for associating the proper signals with the specialcombination of circuits interconnected and for operating the same andfor suppressing the signals which are not appropriate.

for said combination, audit is also necessary to provide means wherebycurrent from the central station may flow in the common-battery circuitsto energize the transmitters located therein, also to prevent thecurrent from passing to the magneto-circuits and to provide separatemeans for energizing the transmitters in the magneto-circuits, and it isalso requisite that means he provided for introducing and withdrawing 'arepeatingcoil'between the conductors of the cord-circuit at the will ofthe operator, so that the same and diverse kinds of circuits may beconductively separated from one another, all of which relates to thewithin-described invention, which will now be described and claimed.

In the drawings which accompany and illustrate the specification, Figure1 is a diagram indicating circuits of diverse character entering acentral-station, together with a switch-cord circuit arranged to uniteany two of either or both kinds for the formation of a circuit forthrough communication. Figs.

2, 3, 4, and 5 are simplified representations of the main connections ofthe switch-cord i placed. At the central station these lines cen-"circuit, and two inagiieto call-circuits, the latter arrangement beingshown both with and without a repeating-coil.

L and L represent common battery telephone-circuits extending betweentheir respective substations A A and a central station 0. They areprovided at the substations with telephones in -a normallyopen bridgebetween the main conductors 1 and 2, and a standard call-bell in serieswith a condenser is connected in parallel with said' telephones, asusual, the regular telephoneswitch being arranged while supporting thereceiving-telephone to maintain the normal discontinuity of thetelephone-bridge, but to close the same when the said receiver is dister in a switchboard, where they are fittedeach with switch-sockets J, aline-signal S, and a normal connection with a source of current cut-0Erelay R each, moreover, being also in E, such as a battery.

L L represent telephone circuits of the older class, technicallycalledmagneto-circuits, which circuits have at theirsubstae tions amagneto-generator to send outgoing signals associated with theirsignal-receiw ing bells and alocal battery to supply cur-v rent fortheir transmitters, the telephoneswitch in the usual manner beingadapted to transfer the line-circuit between the calling appliances andtelephones, according as the receiying-telephone is supported, thereonor v These circuits also are provided with switch-sockets J and haveline-signals S such as annunciators, in a manner well 1 understood.

removed.

L represents one terminal of a toll-circuit, a type of circuit usuallyoperated likewise on the local battery plan.

.0, and its outer-terminal may be fitted with I either a switch-plug'ora switch-socket,as

preferred, at a distant central station.

cuit. comprising the elements of my invention and adapted to form anoperative ,union zb'etween any two circuits of eithertype or between onecircuit of either type and a second of another type, a repeating-coilbe-- I j ing introduced'between the circuits orlnot, as desired "or as,required by the. conditions;

I The figu re shows the several appliances and I circuit arrangementsemployed in carrying I -in series with a condenser h in a bridge 7 out,the 'inv ention for {the purposes hereinbef ore indicated in theirnormal relation'to one another. p

. 1 Band P? arethe. switch plugs of. ordinary structure,having a tipcontact-surface pgand front and rear sleevecontact-surfaces 10 and 19the former-of which is sometimes called I the ring. "These contact-surfac'es 'are adapted to register,- respectively, with correspondingline and local circuit-'contact-suw faces in the plug-sockets. v

S3 is an electromagnetic supervisory signal Itis provided with aswitch-socket J 3 'at the central station resistance r, -Flg. 1, Drepresents a switch-cord cirbetween the front sleeiie or ring conductora and the tip conductor 01' of the plug P.

I is arepeating induction-coil, and k a key controlling the relation oftheswitch-cord conductors c and d with the terminals of the two windingsof the said repeating-coil.

E is the source of signaling and transmitter current'and may. of coursebe common to a number of switch-cords. It has at one pole a connectionto ground G Impedance or retardation coils r r are placed one on eachside of said source in a bridge N between the tip conductor g and frontsleeve conductor f of the plug P R is a relay, which by two armatures 14and 28 or adouble armature controls the con- 'tinuity of the bridge N oris adapted to close or open the same at two different points 31 and 32.vA relay R is included in said bridge when the same is closed andcontrols the op- ;eration of a supervisory signal .9 associated with theplug P v The key It has two contact-springs s s,

p which normally press, respectively, on inner contact-points a: and y,but when the key is operated are separated therefrom and made to pressupon outer contact-points w and 2.

In the former or normal condition of said key there is direct conductivecontinuity between the tip contact-surfaces p of the switchfplugs P andP and alsobet'ween the :ring contact-surfaces 1)? thereof, the samebeing traceable asfollows': between the tips 10 of the plugs byconductor d, key-spring s, inner contact-point 00, condnctor9, junctionm and conductorg, and betweenthe rings g of said @plugs by conductor 0,jiinction point g conductor l2, inner contact-point y, key-spring s andconductor f.v The rear sleeve-contact p of one ofthe plugs (shown asplug P) has :a'conductor 6 extending to the ungrounded pole of thesourceof current E and including the exciting-coils of the relay R and the Therear sleeve-contact of the other plug connects with the un grounded poleof the battery E at 21 by conductor m,which includes theresistance Theobject of the resistancesr and rf is to insure that the current suppliedto the conductors containing them sh'all not-exceed the appropriatestrength. It isto be noted that the socket framesor glands 5 ofthemagneto lines are between the tip-andring conductors of plug P is'toprevent undesired action of the signal S? from the current of' thesource E The connection of the repeating coi1 I, as

hereinbefore indicated, is controlled by the key It. The winding 7; ofthe said coil terminates at one end by conductor 11 in the outer contactto of said key and at the other end in a junction 1 with the ringconductor 0 of the plug P. The second winding j terminates at one end inthe outer contact-pointz of the key and at the other in a junction at 00with conductor 9 leading to the tip of plug P When the key it isdepressed, its springs s s are separated from their normal contacts withpoints my and form new ones with the outer points 20 z. The conductiveunion of the corresponding contact-surfaces of the two plugs is thusdisest'ablished, the two conductors being respectively opened at thekey-points a: and y, and in place of such union a conductive loop is nowformed for each plug be tween the tip and ring contact-surfaces thereof,the repeating-coil windings being included in the said loopsrespectively. Thus the two plugs and substation-circuits,in whoseswitchsockets they may at any moment be inserted, may be inductivelyunited for the transmission and exchange of conversation, while at thesame time the battery-current, which may be bridged to the loop of oneof the plugs, is prevented from obtaining passage to the other, theconductive connection having, as stated, been disestablished. The loopof plug P (the key having been depressed) is traceable from the tip 19by conductor d, key-spriu g s, contact-point w, conductor 11,coil-winding 7;, junction-point g and cord conductor 0 to bring the ringcontact-surface p of the same plug. The loop of plug P similarly may betraced from the tip 19 thereof by conductorg, junction-pointm conductor10, coilwindingj,'condenser 77 conductor 13, outer key-point z,key-spring s and conductor f to the ring contact-surface p of the sameplug.

These changes are simply illustrated by Figs. 2 to 5, inclusive, whereinFigs. 2 and 5 indicate the arrangement of the main-circuit conductorswhen connection is established between two substation-circuits, the keyis remaining unoperated, and the two plugs consequently beingconductively united, while Figs. 3 and 4 respectively represent diversecircuits and similar circuits united by the plug-circuit arrangement,when, the key 7c having been operated, the main conductors are looped tothe .top plugs, respectively, and the connection between the plug-loopsis made inductive only.

The key may be of any well-known selflocking construction, such as thecam-key in extensive use, which when depressed will so remainuntilmanually released, this capa bility being indicated by the groovein its shank, which affords a lodgment for the spring 1 ends.

The bridge N between the main-circuit plug conductors, to whichreference has previously been made as containing the source of current Eand the retardation-coils r and 1*, begins at any point 20 on cordconductor fofl from plug P also.

lay R, conductor 33, the small relay R and conductor 25 to point 27 oncord conductorf. Thus as long as the relay R remains unexcitedthe-circuit-changers 31 and 32 are quiescent and the battery-bridgecontinues closed, supplying current to the main conductors of plug P andany substation-circuit united thereto, and, provided the key it isunoperated, to plug P also. If, however, the said" ikey be operated, theplug P is severed from .all connection with the battery, and when therelay R becomes operative the battery is cht This occurs when the saidplug is inserted into the socket J 2 of one of the magneto-lines whichhave their socketframes grounded direct; but if the plug P be insertedinto a socket J of a common-battery line the current will be so weakenedby the resistance of the cut-oft relay R that while it is suflicientlystrong to operate said cut-oft relay it is too weak to operate thebridge-relay R,Which has relatively few turns or may be stronglyadjusted, and which therefore will not attract its armatures, so thatunder these last-named conditions the batterycurrent will continue to besupplied, as required, through the plug P to the commonbattery circuitswitched thereto.

The small relay R by its armature 18 controls the local circuit ofthesupervisory signal This local circuit extends from battery E by wire17 to the armature 18 and contact-points 34 of relay R and thence to thesignal s and return-wire 16. Any desired reducing resistance r may ofcourse be included therein, or the circuit may be made to include suchcells only of the battery as may be required to furnish the necessarycurrent. In discussing the general operation of the invention it doesnot seem necessary tomake extended reference to the suboperations ofsending and receiving call-signals, since for each of the classes ofcircuit under consideratiou these are performed in the ordinary wayfamiliar to those skilled in the art, and the description will thereforebe restricted to circnit organizations arranged for conversationbetweentwo connected substations. 4 Assuming in the first place that twosubstationcircuits of the common-battery type are thus to be united--asfor example, circuits L and L entering the central station from theirrespective substations A and A and that to this end the plugs Pand Phave been inserted in the switch-sockets J of the said two lines it ismanifest that the ring and tip contact-surfaces of the two plugs willengage the long aud short socket springs a b and that the rear or sleevecontact-surface thereof will register with the socket-frame pieces 15.The branch conductor m leading from the battery E through the resistancer to sleeve 13 of plug P will thus be made IIO to form part of a closedcircuit to ground through cut-off relay R of line L, which, operating,will sever said line from its original terminal and battery connection,and the branch conductor e leading from the same source through relay Rand resistance r to the-sleeve p of plug will in like manner have itscircuit completed through c'ut-ofi relay of line L which performs thesame ofiice for that line. As hereinbefore stated, the on rrent in thislast circuit is under these conditions so regulated that while it issufiiciently strong to work the cut-0E relay it cannot work theless=sensitive relay R. Hence the bridge N between points and 27 of theswitch-cord mainconductors through the local contacts 31 and 32 of saidrelay remains unbroken, and the source of current E is, as long as thetwo lines are united, bridged between the.

the said source E sustains the same relation to line L as it does toline L and supplies current for the transmitter of substation A also.

wherein, though the annunciator-signal S3 remains bridged between thecord conductors c d, it is inert, because a steady battery-current onlyis on the line and is prevented by the condenserh from aifecting saidsignaL' The battery E is shown in its bridge placed between theretardation-coil r on one side and the retardation-coil r andsignal-relay R- on the other, the said devices being allin series insaid bridge. The ground connection G? at one pole of the battery doesnot under these conditions exercise any effect on the arrangement.Ordinarily the signal-relay R is excited and produces the display of thesignal .9 as long as the telephone at either of;

the two substations remains displaced from itsswitch; but,.if desired,the relay may be so adjusted that it becomes operative only when bothtelephones are displaced from their switches and the external resistanceof the battery-circuit through said relaythu's reduced. If desired, thesignal .8 may be used to indicate the moment when the called sub stationA answers by taking his receiver from the switch-hook. This can be doneby transientlyoperating the key it, and therebytemporarily.disconnecting the source E from plug P and'restricting it toplug P. It while,

this-is done the signal .3? appears, th O tor is at once apprised thatthe telephone? receiver atstation A has been takenlfrom j the switchandthe main circuit L 'thereby I the plug P isplaced in theswitch-socket 2 or closed;

cuit of the common or central battery class,

This main-circuit portion of this cir-. cuit arrangement is indicated byFig. 2,.

. rents as these.

' J 3 of the former say circuit L L and the station-circuits with whichthey are associated. Fig. 3 indicates the electrical arrangement of themain circuit under these conditions. That portion of thecord-circuitassociated as a loop with plug P will in all of its operations conformexactly to the description given of the previous arrangement illustratedby Fig. 2. The plug P makes contact by its tip with the'lowersocket-spring b and line .conductor 3 and by its forward sleeve or ring.39 with the upper spring a and conductor 4, and as the plug is forcedhome the said socket-springs are lifted from their normal contacts andthe line-signal S? is out ofi. During conversation the voice-currentsoriginating at either substation are inductively.

this case concerns plug P and substationcircuit L alone and operates tocall for a disconnection when on the conclusion of the message thereceiver is replaced on its hook at substation A, while to indicatethe-disconnecting signal from the stations of lines L or' L in theswitchsockets of which the plug P may be placed the magneto-generator atthe said stations is operated, and the current developed therebyactuates the electromagnetic signal S thecondenser h ofiering no bar tothe passage of said fluctuating our- The third case is one wherein twomagneto circuits are to be interconnected or wherein a magneto circuitis to be connected with a toll circuit. In such a connection one of vtheplugs P is thrust into the switch- -socket J or J of the circuit L or Las the case may be, While the other plug P is inserted into theswitch-socket J 2 of the calledfor circuit L The connections of theregistering contact-surfaces of plug P. have already been described,vbut those of plug P require further notice. Since neitherv of the twocircuits concerned ,in the present case require the use of the centralsource of our- .rent jintheir normal operations of signaling andtalking, the said battery, with' its conis automatically put out of use,this "being accomplished by theoperationofthe relay R,

which opens the battery-bridge at the points 31- and 32. When the-plug Pis inserted in its socket, its main-circuit contact-surfaces pand 19 aswith plug I, register with socketsprings b and a, respectively, and byraising them from their normal contacts actto'cut 01f the line-signal Sand to switch the line conductors 3- 4 intoconnectionwith the cordconductors g f. The third or local cord conductor e at the same time isconnected. directly to ground by the engagement of the sleeve conductor9 of the plug with the grounded socket-frame 5. The current in thecircuit of conductor 6 thus completed is much stronger than in either ofthe former cases,

because the coils of the cut-off relay are no longer included thereinand ,is, in fact, now strong enough to operate relay R, which,attracting its armatures, opens the bridge N at points 31 and 32, sothat for this class of intercommunication the battery, being asuperfluity, is disconnected from the cord-circuit, the relay R and itscontrolled signal 3 are put out of use, and the electromagnetic signal Sis adapted to be operated over both substation-lines from bothsubstations. Under these conditions the main-circuit connections of thecord are represented by the v diagram Fig. 5.

It is sometimes convenient to dispense with the auxiliary groundconnections G and G and in lieu thereof to provide return-conductors ora common return to the complementary battery-pole. This of course is animmaterial change and may or may not be made, as preferred, by thosepracticing the invention. In practice it is, however, sometimesdesirable, even with lineswhich are not energized by the current of acentral and common source, to employ a repeating-coil placedintermediately between the circuits concerned. Obviously this is readilyaccom plished by means of my invention and, as in previous arrangements,requires only the depression of the key It. The cord-circuit mainlineconnections are then as in Fig. 4, andwhen so arranged conversationbetween the substations involved can be satisfactorily exchanged, andthe signal is still responsive to the operation of the call-generatorsat both substations.

As a matter of course switchboard apparatus containing my invention willcomprise as many switch-cord circuits as are necessary for the properconduct of the business of switching lines together.

I claim- 1. In a telephone switchboard apparatus a switch plug and cordcircuit comprising two terminal switch-plugs, each havingcontactsurfaces adapted to register with corresponding contacts in theswitch-sockets of substav tion circuits; two switchboard conductorsnormally uniting the corresponding main-circuit contact-surfaces of saidtwo plugs; a repeating induction-coil; and a key or switch controllingthe relation of said switch-cord conductors, with the terminals of thetwo windings of the said repeating-coil, and adapted when operated tosever the said conductors, and to connect them in two loops from the twoplugs respectively, said loops containing respectively the windings ofsaid repeating-coil, thus dividing the said plugs conductively, anduniting them inductively; substan-- tially as set forth.

2. In a telephone switchboard apparatus, a switch plug and cord circuitcomprising two terminal switch-plugs, each having contactsurfacesadapted to register with corresponding contacts in the switch-sockets ofsubstation-circuits; two switch-cord conductors normally uniting thecorresponding main-circuit contact-surfaces of the said two plugs; arepeating induction-coil; a key or switch controlling the relation ofthe winding-terminals thereof to the said conductors, and adapted tosever the latter, and inter-pose the repeating-coil in such manner thatthe two windings shall be brought into loops of the two plugsrespectively, the said loops being conductively separated butinductively united; a supervisory signal controlled by a relay in abridge of one of said loops; and a second supervisory signal and acondenser in a bridge of the other of said loops; substantially asdescribed.

3. Ina switch plug and cord circuit connection for atelephone-switchboard, the combination of two terminal switch-plugs eachhaving two main contact-surfaces adapted to engage correspondingswitch-socket contacts, and a local contact-surface adapted to engage athird or frame-piece contact of such switchsockets; two switch-cordconductors normally uniting the corresponding main-circuitcontact-surfaces of the said two plugs; a repeating induction-coil; akey or switch for severing said cord-conductors and for uniting the twoends proceeding from the plugs with the two ends of the repeating-coilwindings respectively, thus forming two divisions inductively united bysaid coil; a bridge between the conductors of one of said divisionscontaining a source of current and a signal-controlling relay; a signaldevice controlled. by said relay; a bridge between the conductors of theother division containing an electromagnetic signal device; independentlocalcircuit conductors one for each of the said divisions, leading fromthe said source of current to the local contact-surface of the plug ofits own division; and a relay in the localcircuit conductor of thefirst-named division, controlling the continuity of the bridge thereof,and the connection of the source of current contained therein;substantially as described.

4. In a telephone central-station switchboard apparatus, the combinationwith substation-circuits of two classes, one having its signalingappliancesand telephones supplied with current from a central battery,and the other having substation call-generators andlocaltransmitter-batteries, each circuit having a switch-socket at thecentral station; of

a switch-cord circuit'adapted to switch m,

gether any two circuits of either or both classes, comprising twoterminalswitch-plugs each having three contact-surfaces adapted o a v v7 rogers to register with corresponding contacts i-nathe switch-socketsofv the said circuits,- switchcord conductors extending between saidplugs and unitingthe two forward contact-surfaces of each with thecorresponding contact-surfaces of the other, a bridgecontaining thecentral source-of current uniting the said switch-cord conductors, andmeans for maintaining the continuity and connection of said I bridgewhen one or bothfplugsare. inserted in the switch-socket-srof circuitsof thecommon-batteryclass, and for disconnectingthe same from eitherplug or both plugs, when in ing two terminal switch-plugseach havingsaid circuits, two switch-cord conductors extwom'ain contact-surfacesadapted to engage corresponding switch-socket contacts, and alocal-circuit contact-surface adapted to en-' .gage the frame-contactsof said switch-sockets, two strand conductors uniting the correspondingmain contact-surfaces of the two switch-plugs, a bridge uniting the saidstrand conductors, and containing a centralized source of current, andassociated impedanceco ls, a'th rd strand conductor in a local circultof saidsource leadingthrough the local contact-surface of one of theswitch-plugs,

and when the said plug is inserted in a switchsocket, through the framecontact thereof,

and a relay in 'said local circuit controlling 6. In a telephonecentral-station switch i board'apparatus, the combination ofsubstation-circuits of two classes, one having directly-groundedswitch-socket f raines,and the other having switch-socket framesgrounded through the electromagnet of the cut-01f relay; with aswitch-cord circuit comprising two terminal switch-plugs each havingthree contact-surfaces adapted to register with corresponding contactsin the switch-sockets of tending between said plugs and uniting the twoforward contact-surfaces of each with corresponding ones of. the other,a bridge uniting the said conductors, a centralized source of current,two impedance-coils one on each side of said source, and a signal-comtrolling relay all included in series in said bridge, a supervisorysignal controlled by said relay, a-third cord conductor in a localconnection of a switch-socket wherein such plng'maybe'inseirted, aridarelay'in said local circuit'controlling the continuity of said bridgeand thereby the'con nection of the source of current, and the saidsignal and controllingrrelay; substantially as specified. '7; In atelephone central-station switchboard apparatus, the combination ofsubstation-circuits of two classes, one having directlygroundedswitch-socket frames,and the other-having switch-socket frames groundedthrough the resistance of the cut-off relayexciting coils; withaswitch-cord circuit comprising two terminal switch-plugs eachhavingtwo-main contact-surfaces adapted to engage correspondingswitch-socket contacts, and 'a local-circuit contact-surface adapted toregister with. the frame-piece contact of said switch-contacts, twostrand conductors normally uniting the corresponding main contactsurfaces ofthe two switch-plugs, a repeating induction-coil, akeycontrolling the terininals'of the two windings thereof in relation tothe" said strand conductors, and adapted when operated to sever thecontinuity of said conductors and to form them into andIcontaining thetwo windings-respectively of said repeating-coil, a bridge conductorbetween the strand conductors of one of said loops,including in series acentralized source 1 of current,impedance-coils'and a signal-controllingrelay, a supervisory'signal controlled by'said relay, a third strandassociated with said loop and formingjpart of a local circuit ofsaidsource leading through thelocal contact-surface of the plug of saidloop, and on the insertion of said plug in a switch-socket .through theframe piece contact thereof, and a relay in. said local circuitcontrolling a the continuity ofsaid bridge, and thereby theconnection-of the source of current and thesaid signal and itscontrolling-relay; substantially as specified; r g

8., In a telephone central-station switchboard apparatus, thecombination of substa- 1 tion-circuits of two classes, that is to say 1.common-battery circuits, and magnetocircuits; with a switch-cordcircuit arrangement comprising two terminal switch-plugs I eachhavin gtwo main-circuit contact-surfaces adapted to register with correspondingcon- ;-:tacts in'switch-sockets of the said circuits, i and alocal-circuit contact-surface adapted to egister with the frame-piececontact of said ockets,two strand conductors normally unitng thecorresponding main-circuit contact- I urfaces of the two plugs, arepeating induc- T tion-coil, means for severing the said strand 3conductors and for looping the ends: of the two proceeding from eachplug through the ;two repeating coil windings respectively gtherebyforming the cord main circuit into two inductively-related divisions,abridge i between the conductors of one of the said divisions containinga central source of current, two impedance-coils oneon each side of saidsource and a signal-controlling relay all in series, a supervisorysignal responsive to said relay, a third strand conductor associatedwith said division and forming part of a local circuit of said sourceleadingthrough the local-circuit contact of the plug of said divisionand through the frame-piece contact of a switch-socket wherein such plugmay be inserted, a relay in said local circuit adapted to becomeoperative and to disconnect said source of current and signal-relay whenthe said plug is inserted in a socket of a magneto-line but to maintaintheir connection when said plug is inserted in the socket of acommon-battery line, and an electromagnetic signal device bridged with acondenser between the main conductors of said other division;substantially as and for the purposes set forth.

J. In a telephone central-station switchboard apparatus, the combinationof substation circuits of two classes, that is to say, common-batterycircuits, and magnetocircuits; with a switch-cord circuit arrangementcomprising two terminal switch-plugs each having two main-circuitcontact-surfaces adapted to register with corresponding contacts inswitch-sockets of the said circuits, and a local -circuitcontact-surface adapted to register with the frame-piece contact of saidsockets, two strand conductors normally uniting the correspondingmain-circuit contact-surfaces of the two plugs, a repeatinginduction-coil, means for severing the said strand conductors and forlooping the ends of the two proceeding from each plug through the tworepeating-coil windings respectively thereby forming the cord maincircuit into two inductivelyrelated divisions, a bridge between theconductors of one of the said divisions containing a central source ofcurrent, two impedance-coils one on each side of said source, and asignal-controlling relay, all in series, a supervisory signal responsiveto said relay, a third strand conductor associated with said divisionand forming part of a local circuit of said source leading through thelocal-circuit contact of the plug of said division and through theframe-piece contact of a switch-socket wherein such plug may beinserted, a relay in said local circuit adapted to become operative anddisconnect said source of currentand signal-relay when the said plug isinserted in a socket of a magneto-line, but to maintain their connectionwhen the said plug is inserted in the socket of a common-battery line, athird strand conductor forming part of a similar local circuitassociated with the other division of the said cord-circuit, and leadingthrough the local-circuit contact-surface of the plug thereof, and anyswitch-socket wherein it may be placed, and an electromagnetic signaldevice bridged with a condenser between the main conductors of saidother division; substantially as and for the purposes set forth;

In testimony whereof I have signed my name to this specification, in thepresence of two subscribing witnesses, this 30th day of I March, 1900.

DAVID S. HULFISH.

Witnesses:

SAMUEL G. MCMEEN, THOMAS D. LooKwooD.

